1. Field of the Invention
The present invention relates to a radio transmitting apparatus and a radio receiving apparatus which employ array antenna technology.
2. Description of the Related Art
Array antenna is a technology for performing radio communications using a plurality of antenna elements. According to array antenna technology, the antenna elements are appropriately weighted to electrically control the directivity of the array antenna into a desired direction for thereby reducing interference between paths or users. It is also known according to array antenna technology to improve reception characteristics by giving phase rotation to a plurality of transmission antenna elements.
For an array antenna to operate effectively, it is necessary to equalize in advance amplitude characteristics and phase characteristics of the antenna elements. However, since RF circuits of a radio transmitting and receiving system are analog circuits and analog signals are sent between the radio transmitting and receiving apparatus and the antenna elements, it is not easy to equalize the amplitude characteristics and phase characteristics of the antenna elements. With radio transmitting and receiving systems based on array antenna technology, it is customary to carry out a calibrating process to correct the amplitude characteristics and phase characteristics of the antenna elements.
In recent years, the orthogonal frequency-division multiplexing (OFDM) transmission technique for transmitting a signal with a plurality of orthogonal subcarriers has been adopted by various radio standards. One example of a radio transmitting and receiving apparatus for calibrating the amplitude characteristics and phase characteristics of the antenna elements according to the OFDM transmission technique is disclosed in Japanese Patent Laid-Open No. 2005-348236.
As shown in FIG. 1 of the accompanying drawings, Japanese Patent Laid-Open No. 2005-348236 reveals an antenna array transmitting apparatus including calibration coefficient multipliers 81-1 through 81-B (B represents a positive number), inverse fast Fourier transformers 82-1 through 82-B, radio transmitters 83-1 through 83-B, antenna elements 84-1 through 84-B, radio receiver 91, fast Fourier transformer 92, and calibration value measurer 93.
Calibration coefficient multipliers 81-1 through 81-B multiply transmission baseband signals from branches #1 through #B which have been modulated and multiplied by weights, by calibration coefficients supplied from calibration value measurer 93, and output multiplied results (baseband signals) to inverse fast Fourier transformers 82-1 through 82-B.
Inverse fast Fourier transformers 82-1 through 82-B perform inverse Fourier transform on the baseband signals supplied from calibration coefficient multipliers 81-1 through 81-B, and output the transformed baseband signals to radio transmitters 83-1 through 83-B.
Radio transmitters 83-1 through 83-B convert the baseband signals supplied from inverse fast Fourier transformers 82-1 through 82-B into RF (Radio Frequency) signals, and output the RF signals to antenna elements 84-1 through 84-B and radio receiver 91.
Radio receiver 91 selects one signal, which corresponds to the branch to be calibrated, from among the RF signals supplied from the output terminals of radio transmitters 83-1 through 83-B, converts the selected RF signal into a baseband signal, and outputs the baseband signal to fast Fourier transformer 92.
Fast Fourier transformer 92 performs fast Fourier transform (FFT) on the baseband signal supplied from radio receiver 91, thereby producing a branch-specific reception signal. Fast Fourier transformer 92 outputs the produced reception signal to calibration value measurer 93.
Calibration value measurer 93 compares branch-specific reception signals supplied from fast Fourier transformer 92 with each other to calculate amplitude deviations and phase deviations between the branches, and calculates calibration coefficients based on the amplitude deviations and the phase deviations. Calibration value measurer 93 outputs the calculated calibration coefficients to corresponding calibration coefficient multipliers 81-1 through 81-B.
Calibration coefficient multipliers 81-1 through 81-B shown in FIG. 1 will be described below with reference to FIG. 2 of the accompanying drawings.
FIG. 2 shows conventional calibration coefficient multiplier 81 associated with a single branch.
As shown in FIG. 2, calibration coefficient multiplier 81 includes serial/parallel (S/P) converter 811 and multipliers 812-1 through 812-S (S represents a positive number).
Serial/parallel converter 811 converts a serial transmission baseband signal that has been multiplied by a weight into parallel baseband signals corresponding to a plurality of subcarriers, and outputs the parallel baseband signals to multipliers 812-1 through 812-S.
Multipliers 812-1 through 812-S multiply the baseband signals supplied from serial/parallel converter 811 by calibration coefficients supplied from calibration value measurer 93, and outputs multiplied results to inverse fast Fourier transformers 82-1 through 82-B.
According to the related art described above, calibration coefficients corresponding to respective branches or subcarriers are calculated and used to correct the amplitude characteristics and phase characteristics of the branches for highly accurate calibration.
However, the above radio transmitting and receiving apparatus of the related art is problematic in that since discrete Fourier transform is required for calculating calibration coefficients, the amount of arithmetic operation required to calculate calibration coefficients is large. Therefore, the hardware scale of the radio transmitting and receiving apparatus tends to become so large that it may be difficult to decrease the size of the radio transmitting and receiving apparatus and to install the radio transmitting and receiving apparatus. If the above related art is applied to a portable terminal, then the power consumption of the portable terminal increases and the operating time of the portable terminal that is battery-powered decreases.
The above radio transmitting and receiving system of the related art calibrates all the subcarriers for the respective branches at once. Consequently, it is impossible for the radio transmitting and receiving system to control the calibration period flexible for each subcarrier depending on the need for calibration, and is difficult to reduce power consumption.